We investigate the generation of single-transverse-mode Laguerre-Gaussian (LG) emission from a diode-end-pumped Nd:YVO₄, 1064 nm laser using mode selection via intracavity spherical aberration (SA). We present both theoretical and experimental investigations, examining the limits of the order (both radial and angular indices) of the LG modes which can be produced, along with the resultant output power. We found that in order to generate single-mode emission of low-order LG modes which have relatively small beam diameters, lenses with shorter focal-length were required (to better differentiate neighboring LG modes via SA). The converse was true of LG modes with high-order. Through appropriate choice of the focal length of the intracavity lens, we were able to generate single-mode, LG_0,±m laser output with angular indices m selectable from 1 to 95, as well as those with non-zero radial indices p of up to 4.

Optical vortex laser beams possess a phase singularity and a helical wavefront that twists in the beam’s direction of propagation. Every photon of a vortex beam carries orbital angular momentum (OAM) of $\ell h$, which can be expressed as a phase factor $\exp (i\ell \phi )$ (where $\phi$ is an azimuthal angle).^{1 – 3} The unique characteristics of these beams have enabled new techniques in fields including optical tweezing,^4,5 optical manipulation,^6,7 optical communication,⁸ nano/microfabrication of helical structured materials,⁹ and spectroscopy.^10,11 Laguerre–Gaussian (LG) modes are eigensolutions of the Helmholtz equation under cylindrical symmetry, which can be directly generated from a laser cavity. LG modes are examples of vortex beams. High-order LG modes that have spatial profiles with petal-like features across their azimuth are an important subset of LG modes. These modes have garnered significant attention for their use in manipulation of microparticles,^12,13 spatial measurement,¹⁴ and quantum communication.¹⁵ Therefore, significant research effort has focused on developing methods whereby high-order LG modes with petal-like spatial profiles that can be selectively generated from a laser system.

We report on the generation of an off-axis vortex with tunable non-integer orbital angular momentum (OAM), generated using an idler-resonant mid-infrared optical parametric oscillator (OPO) based on KTiOAsO₄. The design of the OPO enables the OAM of the signal and idler outputs to be varied from 0 to 1 through precise adjustment of cavity length. The maximum vortex output energies at 1.5 and 3.5 μm were 1.95 and 1.1 mJ, respectively, and were obtained at a maximum pump energy of 20 mJ. The means by which the OAM was shared between the signal and idler fields was theoretically examined by modeling the spatial coupling among the pump, signal, and idler fields within the cavity. This work offers a unique means by which exotic vortex laser beams with tunable non-integer OAM can be generated and tuned simply through cavity length adjustments.

We demonstrate a high power (watt-level) self-Raman Nd:GdVO ₄ Laguerre-Gaussian (LG) mode laser by employing an off-axis needle-pumping geometry. The system selectively produces right- or left-handed LG modes by adjusting the off-axis displacement of the pump beam relative to the laser cavity. The maximum output power of the generated 1173 nm LG beam (corresponding to the first-Stokes emission of from the 882 cm ^-1 Raman shift) reaches 1.2 W for an absorbed pump power of 8.6 W. Furthermore, the system enables the generation of a watt-level bottle beam, formed by the coherent superposition of Gaussian and radial LG modes.